In platelet-derived growth factor receptor (PDGFR), tyrosine phosphorylation of specific sites is required for association with known substrates. The tyrosine phosphorylation sites interacting with PI-3 kinase have been identified as tyrosine 731 and tyrosine 742 in human alphaPDGFR. to study the role of PI-3 kinase in the PDGFR-mediated mitogenic signal pathway, both tyrosines were mutated to phenylalanine and it was found that the double mutant receptor (Y731F/Y742F) completely blocked receptor association with PI-3 kinase. anti-P-Tyr-recoverable PI-3 kinase activity was also reduced in PDGF-stimulated cells expressing the double mutant receptor. However, mutation of these tyrosines does not impair PDGF-induced receptor kinase activity or activation of c-raf and MAP kinase protein. Furthermore, DNA synthesis in response to PDGF stimulation was unaffected by these tyrosine mutations in 32D cells. In NIH/3T3 cells, neither single-mutant (Y731F) nor double-mutant receptors impaired foci-forming activity induced by cotransfection with PDGF-AA. These results suggest that PDGFR-associated PI-3 kinase activity is not required for PDGF-induced mitogenesis or transformation signaling.